1. Field of the Invention
This invention relates in general to variable capacitors, and specifically relates to a variable air trimmer capacitor, including a pair of concentric ring electrodes which are movable axially with respect to each other so as to provide a variable overlap of surface areas between the electrodes, thereby varying the capacitance of the capacitor.
2. Description of the Prior Art
Air dielectric trimmer capacitors are commonly used as components in ultra high frequency (UHF) circuits to provide frequency tuning capability. The circuitry involved often is used in mobile equipment which is subjected to thermal stress, vibration, and acceleration which collectively tend to cause the orientation of the capacitor plates to change, thereby altering the capacitance of the device and consequently the frequency to which the circuit is tuned.
A conventional solution to the problems encountered by shock, vibration and temperature is to construct a variable capacitor with a helical threaded engagement between a rotor which contains the movable electrode and an assembly housing which supports the stationary electrode, whereby the rotor is rotated with respect to the stator to vary the overlap of the electrodes. The movable electrode makes electrical contact with the circuit through the rotor, the threads between the rotor and the housing, and the assembly housing, which has an external connection with the circuit. Since helical threads are notoriously poor electrical contacts, it has been normal practice to distort the threads of the rotor in such a manner so as to produce an engagement mismatch in the threads between the rotor and the housing to improve conductivity for the high frequency signal. The thread distortion also helps increase the torque between the rotor and housing so as to minimize frequency drift due to vibrationally-induced migration of the rotor.
Components used in UHF applications, due to the extremely high frequencies involved, are extremely sensitive to current path geometry. Consequently, threads distorted in conventional devices provide a current path which is variable depending upon the position of the thread distortion with respect to the rotor housing external connection. When the capacitor is adjusted to provide minimum capacitance (maximum separation between electrodes), the thread distortion which provides the current path is often located a distance away from the external connection or the rotor housing. In consequence the current path changes dramatically with tuning and thereby changes self inductance, the Q factor, and the self resonant frequency. This provides an additional complication for circuit designers who must treat the variable capacitor as a variable impedance device rather than as a variable capacitor.
The distorted threads of a conventional helical thread connection between a rotor and its housing also provide problems in addition to variable current paths. While distorted thread produces a temporarily improved contact, the device wears at an abnormal rate due to increased friction between the threads. This wear, which is accelerated with repeated tuning, results in a progressively deteriorating electrical contact and a progressively looser rotor fit which is increasingly prone to vibrational migration.
Variable capacitors have a characteristic called "range," which is defined as the specified capacitance over which a capacitive device will tune. For example, a variable capacitor might have a range of 0.8 to 10 picofarads (pF). For such a device, the base or "pedestal" is 0.8 pF and the variable capacitance is 9.2 pF. Circuit designers often are required to employ an external base or "pedestal" capacitor in a circuit in conjunction with variable capacitors so as to obtain the desired basic capacitance for circuit operation together with adjustment capability. While conventional air trimmer capacitors all possess a certain amount of basic capacitance, this capacitance generally is fixed by the geometry of the device, which typically cannot be altered so as to change the basic capacitance. The circuit designer usually selects an external capacitor of appropriate capacitance and provides a parallel circuit connection between the external capacitor and the variable capacitor in order to achieve his design goals. This invariably requires additional components and consequently contributes to increased design and manufacturing costs.